The invention relates to a method for monitoring an exhaust-gas catalytic converter of an internal combustion engine, that is, to a method for monitoring the conversion capacity of an exhaust-gas catalytic converter which is disposed in an exhaust pipe of an internal combustion engine and has a lambda control device, which has a wide-band lambda sensor arranged upstream of the exhaust-gas catalytic converter and controls the air/fuel ratio to a predeterminable set value.
To monitor exhaust-gas catalytic converters for internal combustion engines, there are known methods which use the oxygen storage capacity (OSC method) of the catalytic converter. The storage capacity correlates with the hydrocarbon (HC) conversion in the catalytic converter. If the catalytic converter has good conversion capacities, the lambda fluctuations upstream of the catalytic converter, which are generated by the lambda controller and are recorded by a first lambda sensor, are smoothed by the oxygen storage capacity of the catalytic converter. If the conversion properties of the catalytic converter are reduced or eliminated as a result of aging, poisoning by leaded fuel or by misfires, the control oscillation which is present upstream of the catalytic converter is passed on to the lambda sensor downstream of the catalytic converter. A conclusion can be drawn as to whether the catalytic converter is functional or defective by comparing the signal amplitudes from the two lambda sensors (e.g. German published patent application DE 23 28 459 A1).
In vehicles which are certified in accordance with the stringent ULEV (ultra-low emission vehicle) or SULEV (super ultra-low emission vehicle) limit values, deteriorations in the conversion rate of even a few percent lead to the diagnosis limit values being exceeded. However, relatively low amplitude ratios are determined at these conversion rates. Consequently, it is no longer possible to reliably distinguish between a defective catalytic converter and a functional catalytic converter, in particular taking account of series production scatter and the nonlinear correlation between oxygen storage capacity and HC conversion. Moreover, in the future catalytic converter monitoring for the emission of nitrogen oxides (NOx) will also be demanded.
An NOx sensor offers the possibility of drawing conclusions both as to the HC conversion properties and as to the NOx conversion properties of a catalytic converter from the measured NOx concentration. Our earlier, commonly assigned application No. 10/042,105 (German patent application DE 199 31 321) describes a method for monitoring a three-way exhaust-gas catalytic converter wherein the signal from an NOx sensor arranged downstream of the catalytic converter is evaluated while the internal combustion engine is warming up. For this purpose, a sum value is formed from the measured NOx concentration from the beginning of the time at which the internal combustion engine is started up until a time at which the NOx concentration falls below a predetermined threshold value, and the sum value is compared with a predetermined threshold value. If the threshold value is exceeded, it is concluded that the exhaust-gas catalytic converter has aged. This method therefore describes a light-off diagnosis for the catalytic converter.
It is accordingly an object of the invention to provide a method of monitoring the exhaust gas catalytic converter of an internal combustion engine, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides an accurate method for monitoring the conversion capacity of an exhaust-gas catalytic converter which makes do without expensive temperature sensors and without a second lambda sensor, and provides reliable results in steady-state operation of the internal combustion engine.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of monitoring a conversion capacity of an exhaust-gas catalytic converter of an internal combustion engine, wherein a lambda control device, having a wide-band lambda sensor disposed upstream of the exhaust-gas catalytic converter, and regulating an air/fuel ratio to a predeterminable set value, the method which comprises:
in steady-state operation, and with the internal combustion engine at operating temperature, increasing an oxygen loading of the exhaust-gas catalytic converter to a predetermined value during a diagnosis time by a control intervention of the lambda control device;
recording a NOx concentration in an exhaust downstream of the three-way exhaust-gas catalytic converter with a NOx sensor during the diagnosis time;
determining a steady-state diagnosis value from the values of the NOx concentration during at least one lambda controller oscillation;
comparing the diagnosis value with a predetermined threshold value; and
if the threshold value is exceeded, concluding that the exhaust-gas catalytic converter has aged.
The present invention is based on the idea of monitoring an exhaust-gas catalytic converter by using an NOx sensor for determining the NOx concentration during steady-state operation of the internal combustion engine downstream of the exhaust-gas catalytic converter which is to be diagnosed.
In this method, with the aid of a lambda control device, which has a wide-band lambda sensor arranged upstream of the exhaust-gas catalytic converter and controls the air/fuel ratio to a predeterminable set value, in steady-state operation, with the internal combustion engine at operating temperature, the oxygen loading of the exhaust-gas catalytic converter is increased to a predetermined value during a diagnosis time by a control intervention of the lambda control device. Then, during the diagnosis time the NOx concentration in the exhaust pipe downstream of the three-way exhaust-gas catalytic converter is recorded by means of the NOx sensor, and a steady-state diagnosis value is determined from the values of the NOx concentration during at least one lambda controller oscillation. The diagnosis value is compared with a predetermined threshold value, and if the threshold value is exceeded, it is concluded that the exhaust-gas catalytic converter has aged.
To increase the accuracy of diagnosis, it is advantageous to evaluate not just one controller oscillation, but rather a number of these oscillations, and then to form the mean.
It is possible for both half-waves of the lambda controller oscillation or only the positive half-wave (xe2x80x9clean half-wavexe2x80x9d), during which the signal from the lambda sensor indicates a lean air/fuel mix, to be evaluated.
In accordance with an added feature of the invention, the diagnosis value is determined according to the following relationship:       D    stat    =            1              n        periods              ⁢                  ∫                  t          start                                      t            start                    +                                    n              periods                        ·                          t              periods                                          ⁢                        "LeftBracketingBar"                                    C              NOx                        ⁡                          (              t              )                                "RightBracketingBar"                ·                              C            weighting                    ⁡                      (                          MAF              ,              N                        )                          ·                  ⅆ          t                    
where
Dstat is the diagnosis value;
nperiods is a number of complete periods of the lambda controller;
tperiod is a period time of a complete lambda controller oscillation;
tstart is a start of diagnosis;
cNOx is the NOx concentration; and
cweighting is a weighting factor.
Alternatively, the diagnosis value is determined according to the following relationship:       D    stat    =            1              n        periods              ⁢                  ∫                  t          start                                      t            start                    +                                    n              periods                        ·                          t              periods                                          ⁢                                    "LeftBracketingBar"                                          C                NOx                            ⁡                              (                t                )                                      "RightBracketingBar"                    ·                                    C              weighting                        ⁡                          (                              MAF                ,                N                            )                                ·                      θ            ⁡                          [                                                U                  th_bin                                -                                                      U                    nox_bin                                    ⁡                                      (                    t                    )                                                              ]                                      ⁢                  ⅆ          t                    
where the additional factors are:
Uthxe2x80x94bin a lean threshold for the binary signal of the NOx sensor;
Unoxxe2x80x94bin a binary signal from the NOx sensor; and       θ    ⁡          (      x      )        =      {                            1                                                    for              ⁢                              xe2x80x83                            ⁢              x                        ≥            0                                                0                                                    for              ⁢                              xe2x80x83                            ⁢              x                         less than             0                              
xe2x80x83is a heaviside function.
In accordance with again another feature of the invention, the method comprises choosing for the predetermined value an oxygen loading at which there is as yet no noticeable rise in the NOx concentration downstream of the exhaust-gas catalytic converter for a new exhaust-gas catalytic converter or for a borderline catalytic converter that is still classified as functional.
In accordance with a further feature of the invention, the value is predetermined experimentally and stored in a memory device of a control device controlling the internal combustion engine.
In accordance with again a further feature of the invention, the diagnosis value for steady-state operation of the internal combustion engine is linked to a diagnosis value while the internal combustion engine is warming up, to form a total diagnosis value.
In accordance with again an additional feature of the invention, the total diagnosis value is formed according to the following relationship:                               D                      light            -            off                                    S                      light            -            off                              ·              F                  light          -          off                      +                            D          stat                          S          stat                    ·              F        stat              =      D    total  
where
Dtotal is the total diagnosis value;
Dlight-off is a diagnosis value for a known diagnosis method while the internal combustion engine is warming up;
Slight-off is a diagnosis threshold for a method of this type; and
Flight-off, Fstat are weighting factors
In accordance with a concomitant feature of the invention, the total diagnosis value Dtotal is compared with a threshold value, and a fault indicator device is activated when the threshold value is exceeded.
The method has the particular advantage that it allows diagnosis of catalytic converters with a high degree of accuracy even if stringent exhaust-gas limit values are to be observed.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in method for monitoring an exhaust gas catalytic converter of an internal combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.